The present invention relates most generally to semiconductor waveguide devices, and methods for forming the same.
In recent years it has become practical and cost-effective to fabricate waveguide devices from silicon or other semiconductor substrates. Semiconductor waveguide devices consist of a waveguide layer interposed between cladding layers. Light travels along the waveguide layer and is therefore confined between the cladding layers. This is so because the waveguide layer and the cladding layers are chosen such that the refractive index of the waveguide layer is greater than that of the cladding layers between which the waveguide layer is disposed. Generally, the waveguide structure includes a mesa structure formed on a semiconductor substrate and the mesa structure includes the waveguide layer. Above and below the waveguide layer are cladding layers. The upper cladding layer forms part of the mesa structure while the lower cladding layer may form part of the mesa structure and/or part of the base structure above which the mesa structure extends. Since the waveguide layer is formed within a mesa, it is laterally exposed to air which also includes a refractive index being less than that of the waveguide layer.
The ability of the semiconductor waveguide device to confine light depends on the materials used and the physical surroundings of the waveguide layer. Semiconductor waveguide devices are typically fabricated from a waveguide layer which is clad or bounded by dielectric or other materials with lower indices of refraction than the waveguide layer. This allows the light to propagate within the waveguide with very little attenuation due to the confinement of the light waves by total internal reflection. The light beam-confining ability of the waveguide device therefore depends upon the differences of the refractive indices between the cladding layer and the waveguide layer. The beam confming qualities of the waveguide layer are important because they determine the far field characteristics of the waveguide device which, in turn, affect the optical coupling ability of the waveguide device. Semiconductor waveguide devices may be adapted for various means of optical coupling and may serve various applications such as a light absorbing function or a beam expanding function. As such, the far field and degree of beam confinement required, may vary depending on the application of the semiconductor waveguide.
For any given set of materials having a fixed set of respective refractive indices, then, the beam confining ability of the semiconductor waveguide device depends upon the physical arrangement of the waveguide layer and the adjacent cladding layers. Examples of various semiconductor waveguide structures are shown in FIGS. 9-12. In FIGS. 9-12, waveguide layer 101 is formed having a commonly used cladding layer, InP, above and below waveguide layer 101. The shape and relative position of waveguide layer 101 with respect to mesa structure 103 and base surface 105, influence the beam-confming characteristics of the waveguide device for a given set of materials. The waveguide device is conventionally formed by providing a series of films such as waveguide layer 101 and the upper InP cladding layer, over a substrate or base layer of another cladding material such as InP. Thereafter, a pattern is formed using a masking material 109 and an etch process is used to remove a succession of films from unprotected or exposed regions. It can be understood that the point at which the etch process is terminated, determines where waveguide layer 101 will be disposed relative to the mesa structure 103 formed. For example, in FIGS. 9 and 10, waveguide layer 101 is part of the mesa structure and is bounded laterally by air. In FIG. 11, the etch process is stopped so that waveguide layer 101 forms the base surface 105 above which mesa 103 extends. It can be further seen that in FIG. 12, the etch process is stopped prior to exposing waveguide layer 101, and therefore mesa structure 103 only includes portions of the upper cladding layer InP.
The various types of structures formed and the various dimensions of the structures formed, such as height 107 of waveguide layer 101 above surface 105 as shown in FIG. 9, and depth 117 of waveguide layer 101 beneath base surface 105 as shown in FIG. 12, determine the beam-confining qualities of the semiconductor waveguide device. For example, a light beam being propagated through waveguide layer 101 of the structure shown in FIG. 9, would likely be a tightly confined beam. In contrast, a light beam being propogated through waveguide layer 101 of the structure shown in FIG. 12, would be an unconfined, or weakly confined light beam. The structures shown in FIGS. 10 and 11 provide other beam-confining qualities. Depending on the intended application, various beam-confining abilities may be desired.
The etching process commonly used to form the mesa structures as shown in FIGS. 9-12, includes a mixture of CH4/H2 as etchant gases. This etch gas chemistry is preferred because of its ability to etch the various cladding layers and waveguide layers used to form semiconductor waveguide devices. Because of this ability, a single etching process can be used to form the entire mesa structure by sequentially etching the films. The etch gas chemistry of CH4/H2 is most highly favored, however, because of its ability to produce mesa structures having relatively vertical sidewalls. Mesa structures produced using the CH4/H2 etch gas chemistry, commonly include sidewall angles of 88xc2x0 to 90xc2x0 with the horizontal.
One shortcoming associated with the use of the CH4/H2 etch chemistry to sequentially etch a series of films as shown in FIGS. 9-12, is that the stopping point of the etching process cannot be accurately controlled. Regardless of the particular waveguide structure sought, it is important to accurately and precisely end the etch process at the appropriate time so as to form the desired structure.
The conventional method for xe2x80x9cendpointingxe2x80x9d the etching process is simply to etch for a given time since all of the films will etch in the CH4/H2 etch chemistry used. Due to localized and substrate-to-substrate variations in the various films""etch characteristics, and localized and run-to-run variations of the conditions within etch reactors, it is extremely difficult to predetermine an etch time which will reliably produce the particular waveguide structure desired and do so repeatedly. Once a series of films is formed such as a cladding layer formed over a waveguide layer formed, in turn, over an underlying cladding layer, and an etch process is carried out, any of the various structures shown in FIGS. 9-12 may result for a given etch time, due to etch rate variations as noted above. The structures produced may vary on a run-to-run basis or they may vary within a single substrate.
As such, it can be seen that there is a need in the art to provide an etching process which has an accurately controllable stopping point, and which therefore produces a semiconductor waveguide structure having the physical configuration desired for the specific waveguide application intended.
To address these and other needs, and in view of its purposes, the present invention provides a method for forming a semiconductor waveguide device which utilizes an etch stop layer of InAlAs. The etching chemistry includes a gas mixture of CH4/H2 which sequentially etches a series of films which form the various waveguide layers and cladding layers above the InAlAs layer. The etching process is used to produce a mesa structure used as a semiconductor waveguide device. The relative placement of the InAlAs layer with respect to the cladding layer or layers and the waveguide layer formed above it, determines the mesa structure and, hence, the waveguide structure formed above the InAlAs, because the InAlAs material does not etch in the CH4/H2 etching chemistry used to etch the cladding layers and waveguide layers. Another aspect of the present invention is the waveguide device formed, as above, which includes a structure having physical dimensions which may be accurately predetermined and which therefore can be reliably produced to include desired beam-confining qualities.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but not restrictive, of the invention.